The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In recent years, a rise in oil price causes dashing into unlimited competition for enhancing fuel efficiency.
As a result, research into reduction of weight and the enhancement of fuel efficiency through downsizing is conducted in the case of an engine and research for simultaneously securing operability and fuel efficiency competitiveness through multiple speed stages are conducted in the case of an automatic transmission.
However, in the automatic transmission, as the number of speed stages increases, the number of internal components increase, and as a result, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train which may bring about improved efficiency with a small number of components may further increase fuel efficiency through the multiple speed stages.
In this aspect, in recent years, 8-speed automatic transmission tends to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
Since a span of gear ratios of the recent 8-speed automatic transmission is 6.5 to 7.5, the 8-speed automatic transmission has no great effect for improving fuel efficiency.
In addition, since step ratios between speed stages may not be increased or decreased linearly in a case in which a span of gear ratios of the 8-speed automatic transmission, driving efficiency of an engine and drivability of a vehicle may be deteriorated.